How to model a plastic food tray?
Try to model only one half of the object, then enable the Mirror Modifier and check the X and Y boxes under Axis option (remember to have the origin point placed as pictured below). Apply the modifier when you're done with modeling. You may then add the Subsurf and Solidify Modifiers.



You can easily model the curved corners of a tray using the Spin tool. To make the ribs, divide the surface using Loop Cut tool (Ctrl+R). Select one face, press E and then S to scale it a bit.


Now select the next face and press Shift+R to repeat the earlier operation. Do it with all the faces.


Select all faces and extrude them (E)


You may scale them horizontally for better effect. Scale one, click the next one and use the Shift+R shortcut to speed up the process.


Use a Loop Cut tool again to make the edges of the ribs sharper.